tap, means gently hit it with a hammer, not hard but firmly.
If it gets you started. you have a bad starter solinoid, (the small cylinder on the starter) and you could get away with just replacing that. But for the cost of a rebuild ($120) I'd just have it rebuilt once you know that the starter is the problem.
First what is the voltage that you read across both batteries? (positive of the front to the negative of the rear) Should be ~25volts
If the voltage is low, unhook the batteries and charge them both up. Wouldnt hurt to have the batteries load tested at a local parts store, to rule out a bad cell.
If the voltage is good, first check the connections from the batteries to the buss bars on the passenger side of the firewall, and also the large cables going to the starter.
If the cables and connections look good. Pull the starter and take it to a alternator/starter shop (NOT autozone or the like) and have them test it. They will be able to tell you if the starter is good, if it checks out then there is a wiring issue to be dealt with.